Convergence problem for the surface complexation model

Dear Sir or Madam,

Sorry to disturb you. I am Lang. I created a 1D transport model with surface complexation. I found it could not achieve convergence, while it worked if I attempted to increase the S_AREA (surface area for sorption). Is there anybody can explain me what is wrong?

Additionally, I am not clear about the difference between AMIN (initial specific mineral reactive surface area) and S_AREA (surface area for sorption). If I only know the value of specific surface area for the specific mineral associated with sorption, which one should this  value be (AMIN or S_AREA)? Thank you!

Best regards,

Lang